Filter units are generally used for cleaning air from macroscopic and microscopic contaminations such as dust, microbes and viruses. An airflow is directed through the filter unit in such a way that undesired contaminations are retained by the filter unit and sterile air containing a reduced number of contaminations or no contaminations at all exits the filter unit and is directed into clean/aseptic/sterile working environment or a clean room requiring sterile air.
Filter units for supplying sterile air to sterile working environments are commonly used in medical facilities or in automotive, electronic, pharmaceutical and biotechnology industries or even in the food packaging industry, just to name a few.
Filter units exist in a large variety of solutions and new developments are ongoing in order to improve the quality of the filter units themselves, and, accordingly, to improve the quality and the sterility of the air exiting the filter units.
Filter units are known which comprise one single High-Efficiency Particulate it (HEPA) filter layer or one single Ultra-Low Particulate Air (ULPA) filter layer. These solutions are commonly used for the most stringent applications, including e.g. applications in the food packaging industry, in particular for allowing aseptic production conditions, where sterile working environments are required to increase shelf-life stability of the packaged product and, consequently, consumer safety.
Recently, filter units comprising at least two filter layers have been developed to increase filter efficiency and to guarantee that, even in the event of damage of one filter layer, the air entering the clean/sterile working environment is sterile. This is now one of the requirements laid down by FDA for sterile working environments in the food industry.
It has to be considered that from time to time the filter units need to be sterilized themselves in order to devitalize contaminants such as microbes, viruses and spores filtered out of the air by the filter layers and trapped in these filter layers so that the required quality of the filter units and their respective filter efficiencies can be re-established. Therefore, the design of filter units also needs to consider adequate and efficient means and ways of sterilization.
An example of a known air filter unit comprising respective sterilization means is described in EP2049227. This filter unit comprises a blower configured to direct an airflow, heated by an integrated heating device, through two filter layers; the filter unit further comprises a sterilization layer, which is interposed between the two filter layers and in which liquid or vaporous hydrogen peroxide (H2O2) can be injected to sterilize the filter unit. A pre-filter is also provided at an air inlet to filter out larger contaminations from the airflow entering the unit. During the sterilization of the filter unit, the two filter layers are saturated with liquid or vaporous hydrogen peroxide injected through the sterilization layer and the heating device and the blower are activated. Thus, an airflow is generated through the two filter layers.
The heating device heats the airflow to a temperature above, the activation temperature of the hydrogen peroxide, so that the hydrogen peroxide degrades and reacts on contaminations such as microbes, viruses and spores. Eventually, the blower and the heating device are deactivated once the degradation and, thus, the reaction on the contaminations of the hydrogen peroxide has been initiated. After the sterilization reactions have been terminated, the filter layers can be dried by activating again the blower and the heating device. At the very end, the blower can be activated to remove the degradation products from the filter unit.
Another example of a known air filter unit, in particular for supplying clean/sterile air to a work environment in a food packing factory, is disclosed in US 2009/169421.
This filter unit comprises:                a blower configured to generate and direct en airflow from an air inlet to an air outlet;        a pre-filter arranged at the air inlet for filtering out macroscopic contaminations, such as larger diameter dust, from the airflow entering the filter unit;        two filter layers oriented perpendicularly to each other and through which the airflow coming from the pre-filter passes prior to enter the clean/sterile working environment;        a heater arranged between the blower and the two filters and adapted to adjust the temperature and the humidity of the airflow; and        a chemical gas agent eject apparatus configured to inject gaseous hydrogen peroxide into the airflow upstream of the two filters and downstream of the heater.        
In the example embodiment described in US 2009/169421, the first filter layer crossed by the airflow includes a HEPA filter, whilst the other filter layer includes an ULPA filter.
The operation of the filter unit shown in US 2009/169421 is similar to that described with reference to the filter unit of EP2049227. Even in this case, during sterilization, hydrogen peroxide is supplied through the chemical gas agent eject apparatus into the airflow, which is simultaneously heated to a given temperature.
Air filter units of the type described above are used widely and satisfactorily to supply cleaned/sterilized air to clean/sterile working environments. Within the industry, however, of demand for further improvements is felt, particularly in view of the continual request to increase the filter performance as well as the sterilization efficiency and reliability of the known air filter units.